1. Field of the Invention
The present invention relates to a light-emitting device having a light-emitting element.
2. Description of the Related Art
A display device including a light-emitting element (hereinafter, referred to as a light-emitting device) has advantages such as wide viewing angle, low power consumption, and rapid response speed as compared with a liquid crystal display device, and research and development thereof have been actively carried out.
The light-emitting element has a structure of a light-emitting substance provided between a pair of electrodes, and light from the light-emitting substance is extracted depending on the light-transmitting property of the electrodes.
For example, in the case of desiring to extract light toward one direction, a light-transmitting material is used for one electrode provided in the direction where light is extracted whereas the other electrode is formed by using a material that has no light-transmitting property, that is, a reflective material. The light extraction efficiency can be increased by using reflection at the other electrode effectively.
In addition, a display device is proposed, where the optical distance between a pair of electrodes is made to satisfy a given formula, a resonator structure is introduced, and the resonance wavelength coincides with the peak wavelength of a spectrum of light to be extracted (see Reference 1: Japanese Patent Application Laid-Open No. 2004-178930).
However, an anode is used as a reflective electrode in a top emission panel as shown in Reference 1; therefore, it is desirable to use a material with high reflectivity in visible light such as aluminum or silver. Since silver is difficult to process, aluminum is used as an anode in many cases.
However, a material with high reflectivity such as aluminum or silver is easily oxidized and gets close to an insulator property after oxidizing; therefore, contact resistance thereof gets high. In particular, in a case of forming a layer containing oxygen over a conductive film of aluminum, silver, or the like, the conductive film is easily oxidized. Therefore, contact resistance thereof gets high and consequently current cannot be supplied enough; thus, drive voltage has increased.
Thus, it is an object of the present invention to provide a light-emitting device in which, even when a material with high reflectivity in visible light such as aluminum or silver is used for an electrode, a layer containing oxygen can be formed over the electrode without increasing contact resistance and a manufacturing method thereof.
In view of the above problem, the present invention has a feature of an electrode composed of a stacked structure where a conductive film with high reflectivity that is easily oxidized such as aluminum (Al), silver (Ag), an alloy containing aluminum (Al alloy), or an alloy containing silver (Ag alloy) is stacked with a conductive film composed of a refractory metal material provided in the upper layer of the conductive film.
In addition, a light-emitting device according to the present invention includes a light-emitting element where a mixture layer having a metal oxide, which is a layer containing oxygen, is provided over a conductive film made of a refractory metal material. In other words, a feature of the present invention is to form a mixture layer having a metal oxide in contact with a conductive film made of a refractory metal material which is one of the electrodes having a stacked structure. Note that the conductive film made of a refractory metal material may be formed using a material that is not oxidized due to the mixture layer having a metal oxide or, even when a material is oxidized, any material is acceptable as long as conductivity is shown. In addition, the conductive film made of a refractory metal material can be described as a material having low contact resistance with a metal oxide. As for a material of such a conductive film made of a refractory metal material, titanium (Ti), tungsten (W), molybdenum (Mo), or a compound having these materials, for example, titanium nitride (TiN) which is a titanium compound can be used.
In such a light-emitting element according to the present invention, a transparent electrode material such as indium tin oxide (ITO) is used for a transparent electrode on the side where light from a light-emitting element is emitted. In addition, besides ITO, a material having reflectiveness with high reflectivity, in other words, a material that does not have a light-transmitting property, the film thickness of which is thinned to such a degree that light is transmitted, may also be used. When a material having reflectiveness is thus used for the electrode on the side where light is transmitted, light from a light-emitting element can be interfered between the electrode formed by thinning the material having reflectiveness and the reflective electrode; thus, efficiency in extracting light outside can be increased.
Moreover, in a light-emitting element according to the present invention, light through a light-emitting layer reflected with the reflective electrode and light emitted directly through the light-emitting layer interfere; thus, efficiency in extracting light outside can also be increased. In order to increase extraction efficiency in such a manner, a film thickness of any one of layer in an electroluminescent layer, specifically, a layer between the reflective electrode and the light-emitting layer is controlled. In addition, the film thickness may be made different depending on light-emission color so as to increase the extraction efficiency.
Hereinafter, a specific mode of the present invention will be shown.
According to one feature of the present invention, a light-emitting device includes a first electrode which is stacked; an electroluminescent layer provided over the first electrode; and a second electrode provided over the electroluminescent layer, wherein the electroluminescent layer includes a mixture layer having a metal oxide on the side in contact with the first electrode, and wherein the first electrode includes a conductive film made of a material having low contact resistance with the mixture layer having a metal oxide. The material having low contact resistance with the mixture layer having a metal oxide is a refractory metal material.
According to another feature of the present invention, a light-emitting device includes a first conductive layer and a second conductive layer included in a first electrode; an electroluminescent layer provided in contact with the second conductive layer; and a second electrode provided over the electroluminescent layer, wherein a mixture layer having a metal oxide is provided in a region of the electroluminescent layer in contact with the second conductive layer, and wherein the second conductive layer is formed of a conductive film made of a material having low contact resistance with the mixture layer having a metal oxide. The material having low contact resistance with the mixture layer having a metal oxide is a refractory metal material.
According to another feature of the present invention, a light-emitting device includes a first electrode having reflectiveness which is stacked; an electroluminescent layer provided over the first electrode; and a second electrode having a light-transmitting property which is provided over the electroluminescent layer, wherein the first electrode includes a conductive film made of a refractory metal material on the side in contact with the electroluminescent layer, and wherein the electroluminescent layer includes a mixture layer having a metal oxide on the side in contact with the first electrode. Titanium nitride can be given as an example of the refractory metal material.
According to another feature of the present invention, a light-emitting device includes a first conductive layer and a second conductive layer included in a first electrode having reflectiveness; an electroluminescent layer provided in contact with the second conductive layer; and a second electrode having a light-transmitting property which is provided over the electroluminescent layer, wherein the second conductive layer is made of a refractory metal material, and wherein a mixture layer having a metal oxide is provided in a region of the electroluminescent layer in contact with the second conductive layer. Titanium nitride can be given as an example of the refractory metal material.
According to another feature of the present invention, a light-emitting device includes a plurality of light-emitting elements, in which red, green, and blue light are emitted, having a first electrode which is stacked; an electroluminescent layer provided over the first electrode; and a second electrode provided over the electroluminescent layer, wherein the first electrode includes a conductive film made of titanium nitride on the side in contact with the electroluminescent layer, wherein the electroluminescent layer includes a mixture layer having a metal oxide on the side in contact with the first electrode, wherein the film thickness of the first electrode is the same in the light-emitting element in which red, green, and blue light are emitted, and wherein the film thickness of the electroluminescent layer, specifically, the mixture layer having a metal oxide is different.
According to another feature of the present invention, a light-emitting device includes a plurality of light-emitting elements, in which red, green, and blue light are emitted, having a first conductive layer and a second conductive layer included in a first electrode; an electroluminescent layer provided in contact with the second conductive layer; and a second electrode provided over the electroluminescent layer, wherein the second conductive layer is made of titanium nitride, wherein a mixture layer having a metal oxide is provided in a region of the electroluminescent layer in contact with the second conductive layer, wherein the film thickness of the first electrode is the same in the light-emitting element in which red, green, and blue light are emitted, and wherein the film thickness of the electroluminescent layer, specifically, the film thickness of the mixture layer having a metal oxide is different.
In addition, according to the present invention, a thin film transistor is connected to a first electrode as a switching element. The thin film transistor has an n-channel type or a p-channel type.
Note that a transparent conductive layer such as indium tin oxide (ITO) may be used instead of a mixture layer of a metal oxide for a layer in contact with the first electrode of an electroluminescent layer. In a case of using a transparent conductive layer, a refractory metal material having low contact resistance with the transparent conductive layer may be used for the upper layer of the first electrode.
In addition, as for a reflective electrode, an aluminum alloy, silver, or a silver alloy may be used instead of aluminum. This is because an aluminum alloy, silver, and a silver alloy are also a metal material having reflectiveness. Moreover, titanium (Ti), tungsten (W), or molybdenum (Mo) may be used instead of titanium nitride. In a case of substituting these materials, as for a side in contact with a mixture layer having a metal oxide, which is the lower layer of the electroluminescent layer, a refractory metal material having low contact resistance with the mixture layer is used for the upper layer of the first electrode.
A stacked structure of an electrode material having high reflectiveness such as aluminum and a refractory metal material such as titanium nitride is used for a reflective electrode of a light-emitting element. Accordingly, high light extraction efficiency can be obtained concurrently with preferable contact resistance between a mixture layer having a metal oxide and the reflective electrode. Consequently, it is possible to provide a light-emitting element and a light-emitting device having high luminance compared with a conventional case where a single aluminum layer or a single titanium nitride, which is a reflective electrode is used.
These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description along with the accompanied drawings.